1. Field of the Invention
This invention relates to a technique for driving a liquid crystal display, and more particularly to an apparatus and method for driving a liquid crystal display wherein a liquid crystal can make a rapid transition from a splay state into a bend state.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) panel controls light transmittance of each liquid crystal cell in response to video signals to thereby display a picture. A liquid crystal display panel of an active matrix type includes a switching device for each liquid crystal cell and thus makes each cell more really adaptive to a moving picture. In the active matrix LCD display, a thin film transistor (TFT) is mainly used as the switching device. Since such an LCD panel can be made with a smaller-device size than existent cathode ray tube displays, the LCD panel has been widely used as a monitor in a personal computer or a notebook computer, as well as, in office automation equipment, such as a copy machine, and in portable equipment, such as a cellular phone or pager.
As shown in FIG. 1, a conventional LCD panel includes a digital video card 30 for converting an analog video signal into a digital video data, a power supply 42 for supplying driving voltages, a data driver 36 for supplying video data to data lines DL of the LCD panel 40, a gate driver 34 for sequentially driving gate lines GL of the LCD panel 40, a controller 32 for controlling both the data driver 36 and the gate driver 34, and a gamma voltage generator 38 for applying a gamma voltage to the data driver 36.
In the LCD panel 40 as shown in FIG. 1, a liquid crystal (not shown) is injected between two glass substrates (not shown). The gate lines GL and the data lines DL are formed on the lower glass substrate in such a manner to be perpendicular to each other. In an area adjacent to an intersection between each of the gate lines GL and the data lines DL, a thin film transistor (TFT) is positioned for selectively applying image data inputted from the data lines DL to a liquid crystal cell. To this end, the TFT has a gate terminal (not shown) connected to the gate line GL and a source terminal (not shown) connected to the data line DL. The drain terminal (not shown) of the TFT is connected to a pixel electrode PIXE of a liquid crystal cell.
The digital video card 30 converts an analog image signal into a digital image signal suitable for the LCD panel 40 and detects a synchronous signal included in the analog image signal. The controller 32 receives a driving voltage representative of the digital image signal in voltage range 0˜3.3V from the digital video card 30. The controller 32 supplies red, green and blue digital video data to the data driver 36. Further, the controller 32 generates a dot clock Dclk and a gate start pulse GSP using horizontal/vertical synchronizing signals H and V inputted from the digital video card 30 to provide a timing control of the data driver 36 and the gate driver 34. More particularly, the dot clock Dclk is supplied to the data driver 36 while the gate start pulse GSP is supplied to the gate driver 34.
The power supply 42 receives a low-level common voltage VCC of 0˜3.3V supplied from the digital video card 30 to create a high level common voltage VDD for driving the liquid crystal cells and a driving voltage for driving the gate driver 34. Further, the power supply 42 converts the supplied low-level common voltage VCC of 0˜3.3V into a high level common voltage of 15V and supplies it to the data driver 36. In addition, the power supply 42 converts the supplied low-level common voltage VCC of 0˜3.3V into a gate high voltage VGH of 20V and a gate low voltage VGL of −5V and applies them to the gate driver 34 for a scanning signal in accordance with a gate scanning clock GSC. A common voltage Vcom of 7V is supplied as a common voltage to a common electrode COME on the upper substrate of the liquid crystal display panel by way of an Ag dot provided at a pad portion of the liquid crystal display panel.
The gate driver 34 includes a shift register for responding to the gate start pulse GSP inputted from the controller 32 to sequentially generate a scanning pulse, and a level shifter for shifting a voltage of the scanning pulse into a voltage level suitable for driving the liquid crystal cells. The gate driver 34 applies a gate high voltage VGH and a gate low voltage VGL to the liquid crystal display panel 40 through the gate lines GL. A scanning pulse with a gate high voltage VGH turns on the TFT, and switches video data supplied from the data driver 36 into the liquid crystal cell during a time interval when the TFT is turned on.
The dot clock Dclk from the controller 32, along with red, green and blue digital video data, is inputted to the data driver 36. The data driver 36 latches the red, green and blue digital video data in synchronization with the dot clock Dclk, and then corrects the latched data in accordance with a gamma voltage Vγ. Further, the data driver 36 converts the respective red, green and blue digital data corrected by the gamma voltage Vγ to respective red, green and blue analog data for application to the data lines DL of the LCD panel 40.
A twisted nematic (TN) mode is generally used in a liquid crystal for an LCD panel. In the TN mode, a twisted angle of the liquid crystal alignment is 90°, and an alignment state of the liquid crystal is changed in accordance with an application of an electric field such that the amount of transmission for light from a back light unit through the liquid crystal is controlled. By controlling the twist of the liquid crystal with an electric field, the amount of light transmitted can be varied along a gray scale. However, using the TN mode for a liquid crystal has the problems of a narrow viewing angle and a slow response speed.
To overcome these disadvantages of the TN mode, it has been suggested that liquid crystals can be used with an in-plane switch (IPS) mode or an optically compensated bend (OCB) mode. The OCB mode of the above-mentioned modes has a wider viewing angle and a faster response speed than the TN mode.
Referring to FIG. 2 and FIG. 3, the LCD panel of an OCB mode includes an upper substrate 10 sequentially provided with a color filter array (not shown) and an alignment film (not shown), a lower substrate 12 provided with a TFT array (not shown) and an alignment film (not shown), a liquid crystal 18 injected into a desired gap between the upper substrate 10 and the lower substrate 12 defined by a spacer (not shown), upper and lower polarizers 14 and 22 arranged respectively on the outsides of the upper and lower substrates 10 and 12, an upper compensating film 16 arranged between the upper substrate 10 and the upper polarizer 14, and a compensating film 20 arranged between the lower substrate 12 and the lower polarizer 22 to compensate a phase of an incident light for increasing a viewing angle.
The alignment films of the upper substrate 10 and the lower substrate 12 are subjected to an alignment treatment in the same direction. The liquid crystal 18 between the upper substrate 10 and the lower substrate 12 maintains a splay state, which is an initial alignment state in accordance with an alignment treatment direction of the alignment film when the voltage of an electric field between the upper and lower substrate is less than a specified voltage Vth. In other words, the liquid crystal molecules are arranged at tilt angles of θ° and −θ° at the surfaces of the upper and lower alignment films, respectively. The tilt angles of the liquid crystal molecules decrease towards the center of the liquid crystal cell such that liquid crystal molecules at the center have an angle of 0°, as shown in FIG. 3.
As shown in FIG. 4, the liquid crystal molecules arranged in the splay state irregularly transmit a light at a voltage V less than a specified voltage Vth. Accordingly, a stain or a flicker effect appears in the picture of an LCD panel for a short time when the liquid crystal molecules move from the splay state to the bend state at a specified voltage Vth.
The liquid crystal molecules having such a splay state transition into a bend state at a voltage more than the specified voltage Vth. The time required for transitioning the liquid crystal molecules from the splay state into a bend state is referred to as “transition time.” The transition of the liquid crystal molecules from the splay state into a bend state requires a transition voltage or specified Vth of about at least 3V, as shown in FIG. 5. The tilt angles of the liquid-crystal molecules at the surfaces of the upper and lower alignment films when in the bend state is ±θ, wherein θ is usually about 5°˜15°. However, the tilt angles of the liquid crystal molecules increase towards the center of the liquid crystal cell such that liquid crystal molecules at the center have an angle of 90°. Liquid crystal molecules in the bend state have a characteristic in which light transmittance linearly decreases as the voltage of the electric field increases between the upper and lower substrates. Therefore, the liquid crystal molecules having a bend state are suitable for implementing a gray scale and thus for realizing a picture in an LCD panel.
As described above, in the OCB mode, the liquid crystal is transitioned from the splay state into the bend state at a voltage greater than a transition voltage Vth and light transmittance is linear with respect to an applied voltage above the transition voltage Vth. When a voltage approximate to the transition voltage Vth is supplied, the transition from the splay state into the bend state for the liquid crystal requires a transition time of tens to hundreds of milliseconds. If it takes a long time to transition the liquid crystal, a stain-like appearance emerges in the picture on an LCD panel. Also, the time when flicker occurs in the picture is prolonged. Therefore, it is necessary to transition a liquid crystal from a splay state into a band state within a short time.